It has nothing to do with past lives or reincarnation. The director simply wanted to create a new movie that could satisfy himself. He wanted jaehwan to essentially level up his acting in order to achieve that movie, and he knew that seonghyeon could bring it out of him. Seonghyeon gaslit and manipulated jaehwan into method acting so extreme that he ended up unable separating reality from fiction.
It has nothing to do with past lives or reincarnation. The director simply wanted to create a new movie that could satisfy himself. He wanted jaehwan to essentially level up his acting in order to achieve that movie, and he knew that seonghyeon could bring it out of him. Seonghyeon gaslit and manipulated jaehwan into method acting so extreme that he ended up unable separating reality from fiction.